In a known optical communication network, digital data are typically transmitted in the form of modulated optical signals. In particular, the digital data to be transmitted are used for digitally modulating an optical carrier, i.e. one or more parameters (amplitude and/or phase and/or frequency) of the optical carrier are varied according to the digital data thereby generating a modulated optical signal. The modulated optical signal may be expressed by the following equation:s(t)=A cos(2πft−θ)=[A cos θ] cos(2πft)+[A sin θ] sin(2πft),  [1]where A is the amplitude of the modulated optical signal, f is the frequency of the modulated optical signal, and θ is the phase of the modulated optical signal. Different types of digital modulations are known, such as for instance phase modulations (PSK, DPSK, QPSK, etc.) wherein θ is varied according to the digital data to be transmitted, and amplitude-phase modulations (QAM, etc.) wherein both A and θ are varied according to the digital data to be transmitted.
The modulated optical signal may be further expressed by the following equation:s(t)=I cos(2πft)+Q sin(2πft),  [2]where I=A cos θ is typically termed in-phase component, while Q=A sin θ is typically termed quadrature component.
At the reception side, the modulated optical signal is typically demodulated for retrieving the original digital data. A known receiver suitable for demodulating the modulated optical signal is the so-called “coherent optical receiver”.
A coherent optical receiver typically comprises an analog portion, an analog-to-digital conversion portion and a digital portion. The analog portion typically comprises an opto-electrical circuit that generates two demodulation carriers cos(2πft) and sin(2πft) having frequency ideally equal to the frequency f of the modulated optical signal s(t), mixes them with the modulated optical signal s(t), processes the result of the mixing and opto-electrically converts it thereby providing at its output the in-phase component I and the quadrature component Q of the modulated optical signal s(t) in the form of two analog electrical signals. Then, the analog-to-digital portion typically performs an analog-to-digital conversion of the in-phase component I and the quadrature component Q, and the digital portion finally processes them for retrieving the digital data originally transmitted.